(a) Technical Field of Invention
This invention relates to thermal transfer printing and, in particular, to a thermal transfer printing receiver sheet for use with an associated donor sheet.
(b) Background of the Art
Currently available thermal transfer printing (TTP) techniques generally involve the generation of an image on a receiver sheet by thermal transfer of an imaging medium from an associated donor sheet. The donor sheet typically comprises a supporting substrate of paper, synthetic paper or a polymeric film material coated with a transfer layer comprising a sublimable dye incorporated in an ink medium usually comprising a wax and/or a polymeric resin binder. The associated receiver sheet usually comprises a supporting substrate, of a similar material, having on a surface thereof a dye-receptive, polymeric receiving layer. When an assembly, comprising a donor and a receiver sheet positioned with the respective transfer and receiving layers in contact, is selectively heated in a patterned area derived, for example-from an information signal, such as a television signal, dye is transferred from the donor sheet to the dye-receptive layer of the receiver sheet to form therein a monochrome image of the specified pattern. By repeating the process with different monochrome dyes, usually cyan, magenta and yellow, a full coloured image is produced on the receiver sheet. Image production, therefore depends on dye diffusion by thermal transfer.
To facilitate separation of the imaged sheet from the heated assembly, at least one of the transfer layer and receiving layer may be associated with a release medium, such as a silicone oil.
Although the intense, localised heating required to effect development of a sharp image may be applied by various techniques, including laser beam imaging, a convenient and widely employed technique of thermal printing involves a thermal print-head, for example, of the dot matrix variety in which each dot is represented by an independent heating element or pixcel (electronically controlled, if desired). A problem associated with such a contact print-head is the deformation of the receiver sheet resulting from pressure of the respective elements on the heated, softened assembly. This deformation manifests itself as a reduction in the surface gloss of the receiver sheet, and is particularly significant in receiver sheets the surface of which is initially smooth and glossy, i.e. of the kind which is in demand in the production of high quality art-work. A further problem associated with pressure deformation is the phenomenon of "strike-through" in which an impression of the image is observed on the rear surface of the receiver sheet, i.e. the free surface of the substrate remote from the receiving layer.
Available TTP print equipment has been observed to yield defective imaged receiver sheets comprising inadequately printed spots of relatively low optical density which detract from the appearance and acceptability of the resultant print. There are basically two types of printing flaws. The first type of printing flaw is due to gaps appearing between the printed image of adjacent pixcels and results in the appearance of regularly spaced flaws. The regularly spaced flaws, conveniently referred to as micro-dots, are believed to result from poor conformation of the donor sheet to the print-head at the time of printing. The second type of printing flaw, conveniently referred to as drop-outs, are irregularly spaced and are believed to result from imperfections in the surface of the receiver sheet. There is a need to eliminate both types of the aforementioned printing flaws and to provide a TTP receiver sheet which exhibits high gloss, opacity and whiteness.
(c) The Prior Art
Various receiver sheets have been proposed for use in TTP processes. For example, EP-A-0194106 discloses a heat transferable sheet having a substrate and an image-receiving layer thereon, with an intermediate layer between the substrate and receiving layer.
The intermediate layer serves as a cushion between the substrate and receiving layer and consists mainly of a resin, such as a polyurethane, polyacrylate or polyester, having a 100% modulus of 100 kg/cm.sup.2 or lower, as defined by JIS-K-6301. Inadequate adhesion between the donor and receiver sheets is observed if the intermediate layer is formed from a resin of higher modulus.
U.S. Pat. No. 4734397 seeks to avoid the production of irregular images resulting from entrapment of dust and non-uniformity of the dye-receptive layer by providing a receiver sheet comprising a compression layer between a substrate and a dye-receptive layer. The compression layer, which preferably comprises a resin, such as polymethylmethacrylate, an acrylonitrile-styrene copolymer, a modified polybutylene-terephthalate or a polyurethane, is applied to the substrate as a coating, for example - as a solution in a mixed solvent comprising dichloromethane and trichloroethylene, at a coverage of at least 2.0 g/m.sup.2 and has an elasticity of less than 500% elongation at break. Preferably, the compression layer exhibits a compression modulus of less than 350 megapascals.
European patent application EP-A-292109 describes the production of high quality prints by the use of opaque molecularly oriented thermoplastic films as a substrate for a receiver sheet. Such films generally contain both voids and particulate solids, for example, finely divided inorganic materials and polymeric materials, for giving the opacity and whiteness.
We have now devised a receiver sheet for use in a TTP process which exhibits high gloss, opacity and whiteness, and overcomes or substantially reduces the aforementioned printing flaw problems.